1. Field of the Invention
This invention relates generally to packaging semiconductor devices, and in particular, to grid array semiconductor packages.
2. Description of the Related Technology
Semiconductor devices such as, for example, integrated circuits have revolutionized the field of electronics by making possible a level of technological sophistication unknown in the days of vacuum tubes and even discrete transistors. An integrated circuit die may comprise, on a small silicon chip, many thousand or even a million or more transistors interconnected together to form complex electronic functions. The complex electronic functions of the integrated circuit chip may require several hundred external connections to a related electronic system.
Simple function integrated circuits have been packaged in ceramic packages for high reliability industrial and military applications and in lower cost molded plastic packages for commercial and consumer products. Recently, very large scale integration (VLSI) integrated circuits have outgrown the connection capacity of either the ceramic or molded plastic packaging systems. The integrated circuit packaging industry has developed more sophisticated packages for VLSI integrated circuits that accommodate the increased number of external connections required to the electronic system.
Several of the VLSI integrated circuit packages having high connection capacity are a plastic pin grid array (PPGA) and a plastic ball grid array (PBGA). The PPGA and PBGA packages differ from the prior molded plastic or ceramic packages in that the PPGA and PBGA are, in effect, miniature multiple layer printed circuit boards having the integrated circuit chip contained within the multiple layers and connected to the various conductive paths of the printed circuit boards. Examples of integrated circuit fabrication for VLSI integrated circuit packages are more fully illustrated in commonly-owned co-pending Patent Application Ser. No. 07/917,894 entitled "Ball Bump Grid Array Semiconductor Packages" by Michael Rostoker, Chok J. Chia, Mark Schneider, Michael Steidl, Edwin Fulcher and Keith Newman, filed on Jul. 21, 1992 and incorporated by reference herein for all purposes.
As used herein, the term "semiconductor device" refers to a silicon chip or die containing electronic circuitry and is more commonly referred to as a "semiconductor integrated circuit". The term "semiconductor device assembly" or "integrated circuit assembly" refers to the silicon die and associated packaging containing the die, including means for connecting to a system circuit board, and internal connections such as bond wires, of the die to the leads.
Referring to FIG. 1, a schematic plan view of the bottom face of a PBGA integrated circuit package 100 is illustrated. A printed wire board (PWB) 102 has a plurality of solder balls 104 attached thereto. The solder balls 104 attach the PWB 102 to a system printed circuit board (not illustrated). The solder balls 104 melt when heated in an oven or by applying sufficiently hot air to the PBGA integrated circuit package 100. This is how the PBGA integrated circuit package 100 is electrically and mechanically connected to a system printed circuit board.
Referring now to FIG. 2, a schematic elevational view of the PBGA integrated circuit package 100 is illustrated. The PBGA integrated circuit package 100 comprises the PWB 102, solder balls 104, plastic or epoxy encapsulation 106 and an integrated circuit die 108. The circuitry of the integrated circuit die 108 connects to conductive paths on the PWB 102 which in turn connect to the solder balls 104.
Referring now to FIGS. 3 and 4, prior art fabrication equipment are illustrated in schematic elevational and orthogonal views, respectively. A solder ball holding template 302 is used to fixedly align the solder balls 104 in a predetermined desired pattern. The template 302 is placed into a solder ball spreading device 300 which causes individual loose solder balls 104 to randomly pass over the template 302. The template 302 has holes 304 adapted to receive the solder balls 104.
A vacuum 306 is placed on one face of the template 302 while solder balls 104 pass over the other face of the template 302. The solder balls 104 are attracted to the holes 304 by the vacuum 306 therethrough. After all of the holes 304 of the template 302 have a corresponding solder ball 104 thereon, the template 302 holding the desired pattern of solder balls 104 is ready for the next step in the prior art fabrication process.
Referring now to FIGS. 5 and 6, schematic elevational views of two prior art fabrication steps are illustrated. A solder flux dispenser 500 feeds solder flux to dispensing needles 502. The dispensing needles 502 are brought proximately near the bottom of the PWB 102 of the PBGA integrated circuit package 100. Controlled amounts of solder flux are dispensed from the dispensing needles 502 onto selected locations on the bottom of PWB 102. The dispensed solder flux contacts the PWB 102 and forms solder flux droplets 504 thereon.
A "pick and place" robotics system removes the solder ball holding template 302 from the solder ball spreading device 300, maintaining a vacuum on the template 302 so as to hold the solder balls 104 in place in their respective holes 304. Numeral 600 generally represents a robotics system that moves the template and positions it over the PBGA integrated circuit package 100. The template 302 is placed so that the solder balls 104 are in substantial registration with the corresponding solder flux droplets 504. The template is lowered onto the bottom face of the PWB 102 where the solder balls 104 come into contact with the solder flux droplets 504. Vacuum is removed from the template 302 and the solder balls 104 remain on the face of the PWB 102 after the template 302 is withdrawn. Thus, the solder flux droplets 504 and solder balls 104 have been placed over a pattern of connection pads of the PWB 102 (not illustrated). The robotic system 600 is a very complex and expensive electromechanical device that is utilized to mass produce PBGA integrated circuit assemblies.
The solder balls 104 are initially attached to the PWB 102 by first placing solder flux onto selected connection pad areas of the PWB 102 which have been adapted to receive the solder balls 104. The PWB 102 and solder balls 104 are next heated to a point where the solder balls 104 melt, mechanically and electrically attaching themselves to the PWB 102 connection pads (not illustrated). After the solder balls 104 have cooled down, the residue solder flux must be removed from the PWB 102. Several types of solder flux have been utilized in the industry, some require harsh chemicals such as, for example, chloroflourocarbons (CFC) for removal. CFCs have been suspected of causing depletion of the ozone layer. Some of the newer fluxes used may be removed by less harmful means than CFCs, however, they still require a cleaning operation and subsequent disposal of the waste therefrom. The solder balls 104 consist of a mixture of lead and tin. Lead has been linked to brain damage in children and is considered a hazardous material by the Environmental Protection Agency (EPA).
What is needed is a less costly and environmentally safe method and apparatus for connecting semiconductor device assemblies (integrated circuit packages) to electronic system circuit boards. Accordingly, it is desirable to provide a method and system for simply and cost efficiently manufacturing semiconductor device assemblies in a more environmentally safe manner and to utilize materials that are substantially less hazardous to people and the environment.